The present invention relates generally to an air-fuel ratio control system for controlling the air-fuel ratio of an air-fuel mixture formed for an engine so as to match to a desired air-fuel ratio.
As is well known in the art, when an engine's exhaust system is equipped with a catalytic converter which includes a ternary or three-way catalyst that catalytically and concurrently effects oxidation of hydrocarbons (HC) and carbon monoxide (CO) and reduction of nitrogen oxides (NOx) to convert these noxious components to harmless components, it is necessary, for effectively increasing the amount of noxious components converted by the ternary catalyst, to control the air-fuel ratio of an air-fuel mixture burned in the engine so as to approach as closely as possible the a stoichiometric air-fuel ratio.
In order to solve the aforementioned problem, an air-fuel ratio control system is proposed which controls the air-fuel ratio of an air-fuel mixture formed by a carburetor of an engine to a desired air-fuel ratio by sensing the concentration of a component contained in exhaust gas of the engine. However, air-fuel ratios which are optimum for the output performance of the engine and which are optimum for increasing the amount of noxious components converted by an exhaust gas purifying device of the engine are different from one another in accordance with operating conditions of the engine. More particularly, when the engine is fed with an air-fuel mixture having a stoichiometric air-fuel ratio during cold conditions of the engine, the driveability of the engine is unstable making it difficult to start the engine and to continue to run the engine after starting.